Optical modulators, and in particular, those capable of operating at high frequencies are the workhorses of telecom industry. They are also of great importance for space and military applications where optical high-speed links between ground and space, or between space-borne objects, are used. In the latter application, the size/mass and power budget parameters can be particularly important. One technique allowing for great performance with respect to these parameters, which has also found use in commercial applications, is the implementation of whispering gallery mode (WGM) configurations of optical modulators. Such optical modulators can operate at tens of GHz and have extremely low RF saturation power (less then 0 dBm).
A commonly known issue with respect to high-frequency modulators is that they are typically phase modulators, while it is the amplitude modulation which is desired for the majority of applications. The type of modulation depends on the relative phase of the beat notes between the carrier and both sidebands, so the phase/amplitude modulation conversion requires the ability to manipulate those phases. Furthermore, for many applications, it is highly desirable to eliminate one of the sidebands, thereby implementing a single side-band (SSB) modulator. Finally, it is sometimes useful to suppress higher-order harmonics, or to separate one or more sidebands from the optical carrier.
U.S. Pat. No. 7,043,117 to Matsko discloses a method of producing SSB modulators based on WGM optical resonators that have nonlinear optical materials in multiple sectors where nonlinear coefficients of two adjacent sectors are oppositely poled. In particular, non-equidistant modes of a WGM cavity are utilized in generating SSB modulation. These non-degenerate whispering gallery modes are produced in a WGM dielectric cavity that is fabricated from two crystal wafers with alternating directions of the crystal axis. Such crystals can be difficult to produce and, consequently, relatively expensive.
As a result, a need exists for a simple and inexpensive way to manipulate sidebands in WGM modulators. More specifically, a need exists for the suppression of high-order harmonics, filtering, and conversion of phase to amplitude modulation in WGM modulators.